1. Field of the Invention
The invention relates to processes for the synthesis of dihydric phenols and more particularly to an improved process for recovering bisphenol-A.
2. Brief Description of Related Art
The dihydric phenol 2,2 bis(p-hydroxyphenyl) propane (commonly referred to as "bisphenol-A") is commercially prepared by condensing 2 moles of phenol with a mole of acetone in the presence of an acid catalyst. The phenol is present in a molar excess of the stoichiometric requirement. During the condensation, a number of by-products are formed which are contaminants of the desired product, bisphenol-A. These contaminants, carried in the product stream from the condensation reaction zone, include water, trace quantities of acidic materials derived from the catalyst, unreacted phenol and acetone and a number of isomers of bisphenol-A.
Conventionally, an early step in separating the desired bisphenol-A from the product stream may involve cooling the product stream to induce crystallization and precipitation of the bisphenol-A in the form of a 1:1 adduct with the excess of phenol present. The crystallized adduct is separated, washed, and the phenol removed by distillation, extraction or steam stripping.
In commercial plants for the synthesis of bisphenols, the production of the bisphenol-phenol adducts and delivery to crystallization coolers is generally continuous or semi-continuous. Over periods of time, the coolers will foul with precipitated adduct, reducing their efficiency. This fouling can occur every 4 to 8 weeks, requiring an interruption in their use for a procedure commonly referred to as "meltout". The procedure comprises passing hot phenol through the cooler system to melt out the deposits of crude bisphenol, which occurs mainly in the coolers.
An alternate procedure for defouling the coolers comprises heating both the coolers and the crystallizers with process solutions containing typically 5.0 to 26.0 percent bisphenol to remove the bisphenol deposits coating the inside of the coolers and the crystallizers. The cooling medium in the heat exchanger is also warmed during the meltout.
The time elapsing between meltouts is dictated-in-part by the efficiency of a preceding meltout and, in part, by operating conditions of the process reaction (rate of product delivery, etc.). The frequency of required meltouts affects, of course, the overall efficiency of a commercial process line and the ultimate cost of the desired bisphenol product.
Further, meltouts are at times not successful, when measured by the "Delta T" factor (temperature difference between the process solution and the cooling medium) which increases as the cooler becomes more fouled with bisphenol-phenol adduct on heat-exchanging surfaces.
We have now discovered that the efficiency of defouling coolers and crystallizers with hot phenol is improved substantially when the phenol contains a bisphenol solubilizing proportion of water. Fewer meltouts are required in a given time period, requiring fewer interruptions of continuous and semi-continuous synthesis of the desired bisphenol.
Water has previously been added to the bisphenol-phenol adduct of crystallization to improve adduct purity. However, the process as described in U.S. Pat. No. 4,950,806 (Imuro et al., 1990) requires a lower operating temperature due to the increased solubility of BPA. Other patents relating to water addition to adduct crystallization include U.S. Pat. No. 4,209,646 (Gac et al., 1980) describes BPA adduct crystallization with 2 to 20% water evaporative cooling. Japanese 83-135832 (Mitsui Toats Aug. 12, 1983) describes water addition to add crystals to control solution density and crystals. Japanese Patent JPO7069951-A recently issued to Idemitsu Petrochemical and Tsukishima Kikai Co. describes the use of phenol mixtures containing 1-20% water to reduce solids build-up in evaporative crystallization pipe through which the vaporized components pass. (Note that evaporative cooling involves the use of a vacuum and removal of water's heat of evaporization to cool and crystallize. In contrast, the present inventors use a separate cooling medium and shell and tube heat exchanger system to cool the process solutions.